1. Field of the Invention
The present invention is generally related to nuclear fuel assemblies and, in particular to an apparatus and method of loading fuel rods into exterior grid cells of the fuel assembly.
2. General Background
Grids used in commercial nuclear fuel assemblies are fabricated with stops that grip the fuel rods in place when the fuel assembly is in the as-built condition. During fuel assembly fabrication the stops are withdrawn away from the centerline of each grid cell to allow free passage of the fuel rod into and through the grid. Keying of the stops to cause them to withdraw from the centerline of their grids is typically performed using a rectangular wire that is inserted into and through the windows of the grid. The wire key extends through the full width of the grid and is rotated so that the thickest cross-section of the key wedges the strip section backward and thus withdraws the stop. The key is machined with cutouts to preclude interference with the fuel rods when in the rotated position. On some grid designs the rectangular wire key cannot be used to withdraw the stops located on the exterior grid strip. This restriction is typically caused by a grid corner design that does not allow for adequate passage of the rectangular wire through the corner window. In these cases, an alternate key design or fabrication process is required. A known design uses a machined bar with small keys attached to it by the use of snap rings. The bar seats on the outside surfaces of the grid and the keys (two turned diameters with flats milled on the larger diameter 180 degrees apart) are lined up with the grid window openings. Another fixture is then used to spring the grid strips to an open position and the keys are rotated to catch under the exterior grid strip edges. Patented devices related to nuclear fuel assemblies and the loading of fuel rods which applicant is aware of include the following.
U.S. Pat. No. 3,933,583 discloses a grid structure having elongated slots intermediate and normal to the longitudinal edges of each of the strips at each intersection of the strips whereby the slots form openings in each corner of each passageway.
U.S. Pat. No. 4,651,403 discloses the use of a plurality of elongated corner posts that extend longitudinally between and rigidly interconnecting top and bottom nozzles to form a rigid structural skeleton of the fuel assembly.
U.S. Pat. No. 4,489,037 discloses an automatic channel box fitting position adjusting apparatus to facilitate positional adjustment of a channel box for fitting onto a nuclear fuel assembly.
U.S. Pat. No. 4,747,997 discloses a nuclear fuel rod loader for pulling fuel rods into a fuel assembly skeleton that has a rod gripper with slots defining fingers.
U.S. Pat. No. 4,664,874, U.S. Pat. No. 4,664,875, and U.S. Pat. No. 4,667,547 disclose fixtures relating to the insertion and removal of top nozzles.
U.S. Pat. No. 4,800,061 discloses the use of a thin-walled tubular member that envelopes a fuel rod during insertion and is then removed after the fuel rod is inserted into the grid.
The known patents do not address the specific problem of loading fuel rods in exterior grid cells. Therefore, a need exists for a method and tool that will simplify the loading of fuel rods in exterior grid cells.